What does it mean?
This generic powertrain/engine diagnostic code generally applies to all OBDII-equipped engines but appears more frequently in certain Mazda and Mercedes Benz vehicles.
The cooling fan module is typically mounted behind the radiator in front of the engine. The cooling fan module is usually controlled by the Powertrain Control Module (PCM) based on inputs wired to it.
The PCM receives these inputs via voltage signals to determine engine coolant and intake air temperatures, air conditioning pressures, and vehicle speed. This code is set if these inputs do not match the normal operating voltages stored in the PCM’s memory, or if the cooling fan operation does not match the predetermined information stored in the PCM.
P0485 is set when abnormal voltages are detected during normal fan operation on the power/ground side of the cooling fan circuit. This can occur during low, medium, or high-speed operation. This also includes vehicles using fan relays to operate the cooling fans. This code only concerns electrical issues (cooling fan circuit failure), including intermittent problems.
Troubleshooting steps may vary depending on the manufacturer, type of cooling fan module, and wire colors.
Related cooling fan fault codes include:
P0480 Cooling Fan Control Circuit 1 Malfunction
P0481 Cooling Fan Control Circuit 2 Malfunction
P0482 Cooling Fan Control Circuit 3 Malfunction
P0483 Cooling Fan Rationality Control Malfunction
P0484 Cooling Fan Circuit Overcurrent
Severity and Symptoms
The severity depends on the failure. With an electrical failure, it may not be considered severe because the PCM can compensate for it, unless the engine begins to overheat. While the PCM compensates, the temperature gauge should be monitored during all driving cycles. Typically, the fans run all the time (100% duty cycle).
Symptoms of a P0485 fault code may include:
- Malfunction indicator lamp illuminated
- Engine overheating
- Air conditioning system not functioning properly
Potential Causes
Generally, the causes for this code to set are as follows:
- Open/short circuit in the fan control circuit power supply between the cooling fan module and the PCM – possible
- Short circuit to ground on the fan control circuit to the cooling fan module – possible
- Faulty cooling fan module – very likely
Faulty PCM – least unlikely
Diagnostic and Repair Procedures
A good starting point is always to search for a Technical Service Bulletin (TSB) for your specific vehicle. The vehicle manufacturer may have a PCM flash/reprogramming to address this issue, and it’s worth checking before discovering you’ve taken a long/wrong path.
Next, locate the cooling fan module on your specific vehicle. The cooling fan module (cooling fans) is typically mounted behind the radiator in front of the engine. Once located, visually inspect the connectors and wiring. Look for scratches, rubbing, bare wires, burns, or melted plastic. Separate the connectors and carefully inspect the terminals (the metal parts) inside the connectors. Check if they appear burnt or have a greenish tint indicating corrosion. Use electrical contact cleaner and a plastic-bristled brush if terminal cleaning is necessary. Let it dry and apply electrical grease where the terminals make contact.
If you have a scan tool, clear the diagnostic codes from memory and see if P0485 returns. If not, the connections were likely your problem.
For this particular code, this is the most common area of concern, as are the relays/connections to the relays.
If the code returns, we will need to test the fans and their associated circuits. Typically, there are 2 wires to each cooling fan module. First, disconnect the wiring harness going to the cooling fan module. With a digital volt ohm meter (DVOM), connect one meter lead to one terminal of one of the fans. Connect the remaining meter lead to the other terminal of the fan. It should not be open or shorted. Check the resistance specifications for your specific vehicle. If the fan motor is open or shorted (infinite resistance or no resistance/0 ohms), replace the cooling fan module or, if available, the cooling fan motor.
If this test passes, use a DVOM to check that you have 12V at the cooling fan module power circuit (red lead to the fan power circuit, black lead to a good ground). With a scan tool that can activate the cooling fans, turn them on. If there is no 12 volts to the fan, repair the wiring from the PCM or relay to the fan, or possibly a faulty PCM.
If that is okay, check that the cooling fan module is properly grounded. Connect a test light to the positive 12V battery (red terminal) and touch the other end of the test light to the ground circuit going to the cooling fan module circuit ground. Using the scan tool to actuate the cooling fan module, check if the light illuminates whenever the scan tool actuates the fan. If the test light does not illuminate, it would indicate a circuit problem. If it lights up, wiggle the wiring harness going to the fan to see if the test light flickers, indicating an intermittent connection.
If all tests have passed so far and you continue to get a P0485 code, it would most likely indicate a faulty cooling fan module, although a faulty PCM cannot be ruled out until the fan has been replaced. If in doubt, seek assistance from a qualified automotive diagnostician. PCMs need to be programmed or calibrated to the vehicle to be installed correctly.